A hydraulic shovel has hitherto been known as shown, for example, in FIG. 1 of the drawings attached hereto. In such a hydraulic shovel, a lower vehicle body 2 that is equipped with a traveling body 1 has an upper vehicle body 3 mounted thereon as turnable, the said upper and lower vehicle bodies 2 and 3 constituting a vehicle body 4. A boom 5 is attached onto the said upper vehicle body 3 so as to be swung upwards and downwards by means of a boom cylinder 6. A forward end of the said boom 5 has an arm 7 attached thereto so as to be swung upwards and downwards by means of an arm cylinder 8. A forward end of the said arm 7 has a bucket 9 attached thereto so as to be turnable up and down by means of a bucket cylinder 10. Thus, the hydraulic shovel has been so configured that each of the boom 5 and the arm 7 may be swung upwards and downwards and the bucket 9 may be turned upwards and downwards, and has been used to perform an excavating operation.
A hydraulic circuit for such a hydraulic shovel is generally designed to supply a pressurized discharge fluid from a hydraulic pump into the boom cylinder 6, the arm cylinder 8 and the bucket cylinder 10 via a boom directional control valve, an arm directional control valve and a bucket directional control valve, respectively, to effect an extension and a retraction operation for each of these cylinders.
While an excavating operation is being carried out with a bucket with each of the cylinders expansion and retraction operated as shown in FIG. 1, it can be seen that if the load of excavation is increased due to the presence of a rock in the ground being excavated, the boom 5, the arm 7 and the bucket 9 will cease moving and will be made incapable of continuing the excavating operation. Then, it will become necessary to effect an extension operation for the boom cylinder 6 by acting on the boom directional control valve to swing the boom 5 upwards, thereby displacing the bucket 9 upwards.
Thus, since if a large load of excavation is encountered, it becomes necessary to operate the boom directional control valve to displace the bucket 9 upwards in the conventional hydraulic circuit, the operating efficiency has hitherto been poor and since the vehicle body is then flapped, the operation has also been burdensome for the operator.
In order to resolve these problems, it has been suggested that as shown in FIG. 1, a relief valve 14 having a low relief set pressure (relief-operating at a low pressure) should be provided in a circuit that connects the retraction pressure chamber 11 of the boom cylinder 6 to the boom directional control valve 12. The boom directional control valve 12 will then be switched from its neutral position a to its retraction position b to supply a pressurized discharge fluid of a hydraulic pump 15 into the retraction pressure chamber 11 of the boom cylinder 6. During an excavating operation, if the excavating load is increased whereby the pressure within the first circuit 13 reaches the above mentioned relief set pressure, the pressure fluid in the retraction pressure chamber 11 of the boom cylinder 6 will be allowed to flow out through the relief valve 14 into a reservoir 16. As a result, a situation may be eliminated in which the boom 5 will no longer be swung downwards, and the boom 5, the arm 7 and the bucket 9 will altogether cease moving.
If such a measure is undertaken, however, the pressure within the retraction pressure chamber 11 of the boom cylinder 6 can only be elevated up to the relief set pressure of the relief valve 14. Then, the force by which the boom 5 is swung downwards will be reduced.
For this reason, problems arise such as the inability to lift up the vehicle body 4 with one end portion 1a of the traveling body 1 serving as a supporting point by swinging the boom 5 downwards to press the bucket 9 against the ground surface and the inability to obtain a sufficient force of excavation if a strong force of excavation is required. Thus, it has been recognized that there is an undesirable limitation in establishing the low pressure for the relief set pressure of the relief valve 14; hence there is an undesirable limitation in enhancing the operating efficiency of excavation.
In an attempt to overcome these problems, a hydraulic circuit has been proposed, as disclosed in Japanese Utility Model Unexamined Publication No. Hei 6-1465, in which the relief set pressure of the above mentioned relief valve 14 should be switched between a high pressure and a low pressure.
If such a hydraulic circuit is adopted, the relief set pressure of the relief valve 14 can be set at a low pressure during an excavating operation to enhance the operating efficiency of excavation. And, if the relief set pressure of the relief valve 14 is set at a high pressure, the vehicle body 4 can be lifted up and a strong force of excavation can be obtained.
However, the relief valve in the above mentioned hydraulic circuit is provided to elevate the relief set pressure to a high pressure with a pressurized discharge fluid from an auxiliary pump that is driven by the engine. Thus, if the engine ceases driving, the auxiliary hydraulic pump will also cease discharging the pressure fluid so that the relief set pressure of the relief valve may become a low pressure.
For this reason, if, for example, the relief set pressure is set at an elevated pressure to lift up the vehicle body 4 and the engine is halted for any cause in that state, the relief set pressure of the relief valve 14 will become a low pressure and this will cause the pressure within the retraction pressure chamber 11 of the boom cylinder 6 to be reduced to a low pressure. As a result, an extension operation will occur in the boom cylinder 6 due to the weight of the vehicle body and so forth so that the vehicle body 4 may be dropped.
More specifically, if a retraction operation occurs in the retraction pressure chamber 11 of the boom cylinder 6 when a pressure fluid is supplied therein, the boom 5 will be swung downwards with a supporting point 5a at a side of the vehicle body serving as a fulcrum. Then, if the bucket 9 is in contact with the ground surface, the boom 5 will not be able to be swung downwards and will, on the contrary, be swung upwards with a supporting point 5b at a side of the arm serving as a fulcrum so that the vehicle body 4 may be lifted up as shown by the phantom line. In that state, if the relief set pressure of the relief valve 14 is reduced to a low pressure, a force of extension due to the weight of the vehicle body 4 will be exerted on the boom cylinder 6 and, as a result, the pressure fluid within the retraction pressure chamber 11 will be allowed to flow out through the relief valve 14 into a reservoir 16. The boom cylinder 6 will then be extension operated, causing the vehicle body 4 to be dropped.
Thus, due to the possibility that the vehicle body may be dropped when the engine is halted, there has been a serious problem as to safety in the prior art.
Also, if the low set pressure of the relief valve 14 is elevated not to cause a drop of the vehicle body 4, a difference with a high set pressure will be reduced, thus giving rise to the problem that an enhancement of the operating efficiency of an excavating operating that is originally sought may not be realized.
With the above mentioned problems taken into account, it is, accordingly, an object of the present invention to provide a hydraulic circuit for a boom cylinder in a hydraulic shovel, which provides a sufficient safety and in which in a state where a vehicle body has been lifted up, there may be no false drop of the vehicle body when the engine is halted.